1. Field of the Invention
The present invention relates to turbine engines and more specifically to the disk and sideplate constructions of turbine wheel assemblies.
2. Description of the Prior Art
A typical turbine wheel assembly in a gas turbine engine comprises a rotor shaft to which one or more disk and blade assemblies are attached. Each disk and blade assembly includes a multiplicity of turbine blades each having a root section which engages a corresponding root cavity at the periphery of the disk member. The root cavities of the disk are oriented in an essentially axial direction and extend through both sides of the turbine disk. A plurality of segmented sideplates is attached to at least one side of the disk by rivets or other suitable means. The sideplates cover the root sections of the blades to axially retain the blades and to prevent the leakage of working medium between successive stages through the interfaces between the blade roots and the disk cavities. In many turbine constructions cooling air is provided through the disk to the root section of each turbine blade and in these constructions the turbine sideplates perform the additional function of axially sealing the turbine root cavities to prevent the premature flow of cooling air into the working medium flowpath.
Each turbine disk is a principal structural member of the wheel assembly. As the disk rotates the turbine blades and sideplates mounted thereon are thrust radially by centrifugal forces until they engage corresponding retaining surfaces on the disk. The walls of the root cavity comprise the blade retaining surface while a plurality of sideplate retaining lugs oppose radial movement of the sideplates. Each sideplate retaining lug has an arcuate surface which faces in a radially inward direction toward the axis of rotation of the wheel assembly and engages a corresponding sideplate bearing surface. A bending moment in the disk lug is developed about a moment arm extending from the base of the disk lug to the point of contact between the sideplate bearing surface and the arcuate surface of the lug. Rotor speeds in modern gas turbine engines commonly exceed 7,500 rpm and in one conventional construction tested generate bending stresses of approximately 58,000 pounds per square inch at the base of each retaining lug of the disk, the stresses being proportional to the length of the moment arm described above.
Although design stresses are always lower than the yield strength of the affected material, fatigue cracks appear after repeated cycling to any given stress value. The point at which cracks begin to appear is known as the low cycle fatigue limit or LCF in which each cycle comprises and aircraft take-off, cruise and landing. At a maximum stress of 58,000 pounds per square inch psi as experienced in the construction tested, the LCF life of the disk is limited to between 1,200 to 1,500 cycles.
Increases in the cycle life of the turbine disk are required to bring the service life of that component into compatible relationship with economic operation of the gas turbine engine. Although cycle life increases can generally be effected by corresponding increases in material thickness, continuing efforts are underway to reduce maximum stresses at the base of each sideplate retaining lug during the take-off condition without undesirably adding to the total engine weight.